The present invention relates generally to the operation of a screw compressor. More specifically, the present invention relates to the variable speed operation of a screw compressor connected to a motor powered by a variable speed drive that can provide the motor an input voltage and frequency greater than the motor's rated voltage and frequency.
The capacity of a screw compressor is based on the amount of gas entering the screw compressor and the rotational speed of the screws or rotors compressing the gas. In the past, the speed of the rotors of a screw compressor was limited to the maximum rated output speed of the induction motor driving the rotors of the screw compressor. This single speed screw compressor would have a single output capacity based upon the rotor speed in the screw compressor. To get a rotor speed greater than the maximum rated speed of the induction motor and a corresponding increase in the output capacity of the screw compressor, a gearing arrangement had to be connected between the motor and rotors of the screw compressor. However, the introduction of the gearing arrangement to obtain increased rotor speeds in the screw compressor resulted in a reduction in the efficiency of the compressor system and increased cost and maintenance for the compressor system. In addition, the screw compressor still only has one output capacity because the rotors are still being driven at a single (albeit faster) speed.
Next, several techniques were developed to obtain a variable capacity in a screw compressor. In one technique, the screw compressor includes a slide valve to obtain variable capacities. The slide valve operates to divert a portion of the gas being compressed by the rotors away from the compressor discharge. Often, the gas diverted by the slide valve is returned to the suction inlet of the compressor. While the incorporation of a slide valve can provide a variable output capacity screw compressor, there are several drawbacks to using a slide valve. First, the slide valve provides a leakage passageway for gas, thereby reducing the efficiency of the compressor. Next, the incorporation of a slide valve in a screw compressor involves a complicated machining process and a corresponding increased cost. Finally, the maximum rotor speed and capacity of the screw compressor are based on the maximum rated speed of the motor and a gearing arrangement would be required to obtain a rotor speed greater then the rated motor speed.
Another technique for obtaining a variable capacity in a screw compressor involves connecting the motor of the screw compressor to a variable speed drive. The variable speed drive can vary the frequency and/or voltage that is provided to the motor to vary the output speed of the motor, which, in turn, varies the speed of the rotors to obtain variable output capacities of the screw compressor. However, most variable speed drives are not capable of providing an output voltage greater than the input voltage. This limitation on the output voltage of the variable speed drive limits the maximum speed of the motor to a speed that corresponds to the speed of the motor operated at line voltage. As discussed above, to obtain greater rotor speeds in the compressor, a gearing arrangement has to be incorporated between the motor and the rotors to increase the output rotational speed of the motor driving the rotors of the compressors.
Therefore, what is needed is a system and technique for variable speed operation of a screw compressor that does not involve a slide valve and can provide increased rotor speeds without the use a gearing arrangement.